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'04 Cavalier, 120K mi. We've had it for 6 years, driven every winter. Now, this winter, we occasionally get an ABS fault. This probably always occurs under slipping conditions. When it happens, the 'Low Trak' light also comes on. So, it would be a 'Yeah, so?' thing, except it didn't happen before, and, I don't see this winter's driving as any different than prior years.

Tires are not new, but more than legal; I would say tread is about 'normal' for us - so, again, not markedly different than prior years. I also swapped them (front <-> rear), which didn't change anything.

Suspension is OK, AFACT, though the rear struts are old. In any case, the fault doesn't correlate with bouncing or such.

Edit (OP): The ABS (always, I think) works normally, and doesn't fault, when the brakes are applied. The fault occurs when there's slip under torque. Even, pretty minimal torque. I don't know if it happens every time there's slip. It seems unlikely, but I can't rule it out.

Q: Why the change?

Bonus Q: Once a fault occurs, ABS is disabled until the car is shut off and restarted. Why would they design it that way? If ABS is safer, isn't this unnecessarily less safe?

Thanks

  • 2
    Bonus Answer: In a system like the brakes, failures are designed to be as safe as possible. ABS is not a requirement, but brakes are. If there is anything wrong with the ABS, it's going to err on the side of caution and disable itself to make sure it doesn't affect the basic braking ability at all. In a system like the air bags (in my VW at least), if any fault is detected the system it is disabled until you clear the code with a dealer scan-tool even if the fault was fixed. – JPhi1618 Feb 15 '16 at 21:30
  • Do you feel the brake pedal vibrate under the slipping condition? – HandyHowie Feb 15 '16 at 21:36
  • Is the ABS light on or is the low trac light the only thing that illuminates during slipping? Does the low trac light shut off after the wheels are done slipping? – Ben Feb 15 '16 at 21:39
  • @HandyHowie - No vibe. I would notice it. – George Feb 15 '16 at 23:45
  • @Ben - ABS is on. Also, 'Service Engine', simultaneously. I'm quite sure 'Low Trac' goes off after slipping stops. – George Feb 15 '16 at 23:48
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It appears that as soon as the vehicle detects a wheel has locked up, the ABS goes into failure mode. This would suggest to me that you may have a fault in the abs module.

If it was a wheel bearing or sensor that was at fault, I would expect occasional false activation of the ABS and also normal activation where the pedal judders.

It looks instead like the ABS ECU is detecting the wheel locking, then it tries to activate the ABS, but either the ABS valve body or the possible the pump mechanism is not working correctly and so the ABS fails.

  • See my edit above - thinking about what you say, it reminded me that the fault doesn't occur when braking - it's when torque applied. – George Feb 16 '16 at 21:37
  • @George The traction control system is pulsing the brakes on rather than pulsing them off as when the ABS functions. The fault could still be one of the solenoid valves in the valve body. – HandyHowie Feb 16 '16 at 22:15
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In my experience intermittent ABS issues are not solely but often caused by a worn wheel bearing. This is speculation but it may be that the spinning wheel is effecting the wheel bearing. The worn bearing allows the hub to move enough to miss a sensor pulse and light the fault light.

  • I do check the FWB's periodically, and I've changed both hubs. (A while ago.) I'll check again when I can. (It's a little unpleasant out right now.) Would the same apply to play in the steering linkage? – George Feb 15 '16 at 23:52

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